Any attempt to outlaw firearms would focus also on its component parts – those that comprise the firearm itself and those that comprise the ammunition and any specialized machinery used to produce these items. Legitimate companies would be limited to producing these items for the police, Armed Forces, etc.
The consequences for possessing firearms and ammunition would have to be severe - lengthy prison sentences starting at somewhere around twenty years. The penalty for smuggling these items across our borders would be even harsher, perhaps even life sentences.
The security of our borders adjacent to neighboring countries and oceans would have to be strengthened far beyond what they are today. A large number of special law enforcement agents would be required, with the power to search any home without warrant or probable cause. The existing police across the country would require a strengthening of their forces. The National Guard and the military would need to be on alert in case of an uprising from pro-gun factions.
Only extreme measures like this would have a chance of keeping firearms out of the hands of criminals and potential criminals.
It is not a likely scenario.
Such a limitation would be just as unconstitutional as poll taxes and grandfather clauses...anything that prevents a person from exercising their rights (unless through their own actions and after a due process hearing) is unconstitutional.